1. Field of the Invention
The present invention relates to an optical feedback control unit for determining an optical output by a feedback control, an optical communication system, a feedback control method, and a recording medium for recording a program.
2. Description of Related Art
In recent years, a transmission technology such as a metropolitan WDM (wavelength division multiplexing) has been come into practical use, and an optical direct amplified transmission has been popularized in multiple node links. Generally, for example, an optical add/drop apparatus, a gain-deviation compensation apparatus (optical equalizer), and a wavelength cross-connect apparatus, etc. are disposed in each node, thereby performing a feedback control such that an optical output of each node is constant.
This feedback control is to compensate an optical level when changing a setting of the optical add/drop or the loss increase of parts over a long-term (several months or several years) span and the loss change of a transmission path when transferring obstruction.
The feedback period is regulated by a response and operation time of devices such as a wavelength blocker, a wavelength selection switch, and an optical channel monitor, or the like that are used in the above-mentioned apparatuses, the feedback period being about several hundreds ms to about several s. In addition, as the feedback control, there is a digital feedback that feedbacks quantity provided by a prescribed calculation in each prescribed period.
Furthermore, as an optical communication system related to the technology of the present invention, OADM (optical add/drop multiplexer) node that monitors a power level by branching off an inserted optical signal in an output unit of an optical amplifier to hold an optical output per channel of the inserted optical signal at a predetermined value being equalized to the optical output per channel of another optical signal is used (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2002-185407).
Furthermore, as the related art performing the feedback control, a continuously variable transmission stopping the feedback control sets a width of a dead zone during transmission to be smaller than that of the dead zone during non-transmission when a deviation between a target transmission ratio or a target rotation number and an actual transmission ratio or an actual input rotation number is within a prescribed dead zone (for example, see JP-A No. 2004-116537).
Hereinafter, the problems of each of the related arts will be described.
First, since the above-described feedback period is the comparable order as the period of an optical level change due to the loss of the transmission path and the polarized variation, unnecessary response behavior is sometimes shown so that it may become one factor making the system unstable.
FIG. 1 illustrates representative variable elements to be occurred in the optical communication system and variation span and variation quantity due to the variable elements. As illustrated in FIG. 1, the variation spans become short by means of the variable elements such as a setting modification of a breakdown or add/drop, but the variation span becomes long in a part degradation or season variation. Preferably, among the variable elements, the feedback control does not respond to the optical level due to the polarized variation or the loss variation but reliably can respond to the other variable elements.
Here, for example, when the feedback period is sufficiently (for example, more than ten times) longer than the variation period of the optical level due to the polarized variation or the loss variation, it can make no response to the optical level change due to the polarized variation or the loss variation. However, in this case, there is a problem that the response with respect to the optical level change which is to be quickly compensated, such as the optical level compensation, etc., during the breakdown or the setting modification is also delayed.
In addition, according to JP-A No. 2002-185407, the optical output per the channel of one optical signal is equalized to the optical output per the channel of another optical signal by the feedback control. Accordingly, there is no consideration to perform the stable feedback control without being affected by the optical level change due to the polarized variation or the loss variation.
Furthermore, according to JP-A No. 2004-116537, there is no consideration about the application to the optical signal and the width of the dead zone is only changed from the feedback control by the operation mode during the transmission or non-transmission so that there is no consideration to perform the stable feedback control in the system to which plural nodes are connected.
That is, in a case where the feedback control is performed in the optical communication system to which the plural nodes are connected, for example, when the fluctuation of the optical signal from the sending terminal occurs in the system configuration illustrated in FIG. 2, the feedback control is asynchronously initiated in each node so as to compensate the fluctuation. Since variable elements increase with forwarding downstream nodes, the stabilization is progressed in the order from an upstream node close to the sending terminal.
Here, the dead zone is uniformly installed irrespective of whether the system is the stabilized status or the fluctuation state as well as even in the state where the fluctuation occurs in the upstream, the feedback control is continuously performed until the width of the dead zone in the state where the system is stabilized.
In the upstream node, when the feedback control is performed until the dead zone in the state where the system is stabilized, the upstream node is stabilized, but there is a risk of increasing the variable elements in the downstream node. Therefore, it takes much time until the overall system is stabilized.
As described above, according to the feedback control of JP-A No. 2004-116537, there is no consideration to make the unstable system stabilize by the proper feedback control without being affected by the optical level change due to the polarized variation or the loss variation in the optical communication system to which the plural nodes are connected.